Improvements in power-electronics and the advent of digital signal processors (DSPs) has allowed for the use of inexpensive and rugged induction motors in applications which were so far reserved for brushed DC motors (such as traction drives and variable speed industrial drives). By taking advantage of the new control technologies and new hardware available, AC induction motors can achieve the same dynamic performance as a DC motor, while exhibiting higher power density, lower cost and zero maintenance.
The two most prevalent advanced control methods for induction motors are “field oriented control” (FOC) and “direct torque control” (DTC)”. FOC was first proposed over thirty years ago (in 1971 by F. Blaschke), while the more “modern” DTC emerged a decade later. The advantage of DTC is its simplicity, requiring no current regulator and being very robust to parameter variations. DTC, however, also presents some drawbacks compared to FOC, such as high current ripple, difficulty to control torque and flux at low speeds, high acoustic noise at low speed and lack of direct current control. These issues are the reason why FOC is still widely used for electric traction, where low speed performance and accurate current control are very important features.
In FOC, the three-phase current wave-forms and voltage wave-forms are transformed into a two-axis dq-frame (synchronous frame) which is rotating at the frequency of the electrical waveforms. By this coordinate transformation, AC waveforms result in DC vectors (also called space vectors). The advantage of this approach is that it is much easier to control DC quantities than AC quantities. The implementation of a digital current regulator is therefore relatively straightforward and can be very robust and dynamic.
If the synchronous frame is aligned with the rotor flux (Ψr), then the current component aligned with the flux (direct current, Id) can be used to control the flux of the machine while the quadrature current (Iq) controls the magnitude of the motor torque. This decoupling of torque and flux allows for dynamic variable speed performance that rivals, if not outperforms, a DC motor.
In order to align the d-q frame with the (rotating) rotor flux, the instantaneous position of the rotor flux (θr) with respect to the motor winding phases U, V, W needs to be known, and therein lays the difficulty of the FOC method. Since it is impractical to measure the position of the rotor flux directly, it has to be estimated indirectly, hence the method is often referred to as indirect field oriented control.
The technique for estimating the position (θr) of the rotor flux is based on integrating the electrical frequency (ωs) which can be computed from the rotational frequency of the rotor (ω) and the slip frequency (ωr):θr=∫ωsdt ωs=ω+ωr ω=Pp·Ω  (1)Pp stands for the pole-pair number of the motor, Ω is the mechanical speed.θr=Pp∫Ωdt+∫ωr dt  (2)
Since the integral of the mechanical speed can be measured (by means of a position encoder), the problem is reduced to the integration of the (estimated) slip frequency ωr. Most often ωr is determined based on the following equation:
                              ω          ⁢                                          ⁢          r                =                              Rr                          Ψ              ⁢                                                          ⁢              r                                ·          Iq                                    (        3        )            Rr stands for the rotor resistance.
The difficulty with the above expression for slip is the fact that it depends on the rotor resistance, which changes with rotor temperature. In the typical use of an induction motor, the rotor temperature experiences a large temperature swing and the rotor resistance will vary accordingly. If the slip is not estimated correctly, the synchronous (dq) frame is no longer aligned with the rotor flux and the FOC becomes “detuned” hence degrading the motor performance.